In the manufacture of semiconductor devices, particularly devices such as thyristors, n-p-n or p-n-p transistors, silicon rectifiers, diodes, silicon solar cells, and the like, metal contacts must be applied to the device to apply or carry away electric current during operation of the device. The substrate layer, which may be overcoated or passivated with layers such as silicon oxide, silicon nitride, or metal oxide-containing glasses, is exposed using standard photolithographic techniques with a suitable resist in the areas to be metallized. The exposed substrate surface is cleaned and the metal is electrolessly plated onto these exposed portions of the substrate surface. Since substrate materials, such as silicon, which may have been variously p- or n- doped during manufacture of the particular device, do not accept electroless plating in a uniform manner, the substrate surface is first sensitized with a noble or other metal. This sensitization layer is very thin and discontinuous and usually forms islands of metal on the surface to be plated. These islands act as seeds or nucleation sites for subsequent electroless plating. After the electroless plating, the metallized substrate is sintered at elevated temperatures to react the metal layer with the substrate to form a strongly adherent film of metal silicide. The plating and sintering steps may be repeated if desired.
In the conventional process for making semiconductor contacts the sensitizing metal solution contains a metal salt, such as palladium chloride or gold chloride and HF in an acidic diluent such as acetic acid. The exchange reaction which takes place in this process at the silicon surface is represented by the following: EQU Pd.sup.+2 +2e.fwdarw.Pd.degree. EQU Si.degree.-4e.fwdarw.Si.sup.+4
Thus the metal, palladium, is deposited on the silicon and the silicon is removed by forming a water soluble silicon fluoride. The concentrations of the HF and palladium chloride in the solution are varied depending on the doping levels of the silicon surface to be sensitized. The HF is required to maintain the silicon surface in an active state, free of silicon oxide deposits, and to remove the ionized silicon as a water soluble silicon fluoride compound which is formed during the exchange reaction.
At the optimum HF concentrations, the solubility of passivating layers of silicon dioxide and metal oxide-containing glass in the sensitizing solution is considerable. This solubility is undesirable for several reasons: the passivating layers can be damaged by the HF; and metal oxides, such as lead oxide, which may be present in the glass passivating layer, are dissolved by the HF and deposit on the exposed silicon surface as ionic or metallic lead, poisoning them to subsequent electroless plating.
Thus, a method of applying sensitizing metal deposits which eliminates the use of HP would be highly desirable.